Vacuum at spindle only 13.8" Hg. Not enough to satisfy the vacuum sensor. Verified incoming supply vacuum- OK at 25" Hg. Verified all pneumatic lines and solinoid- All OK. Isolated loss of spindle vacuum to shaft seal. Replaced the shaft seal. Now vacum at spindle is 21.3" Hg with a wafer and chuck. Tested operation with standard program. All OK now.